There is a need to measure cerebral blood flow during several medical events and procedures, because any disturbance to the flow of blood to the brain may cause an injury to the function of the brain cells, and even death of brain cells if the disturbance is prolonged. Maintaining blood flow to the brain is especially important because brain cells are more vulnerable to a lack of oxygen than other cells, and because brain cells usually cannot regenerate following an injury. A number of common situations may cause a decrease in the general blood flow to the brain, including arrhythmia, myocardial infarction, and traumatic hemorrhagic shock. In such cases, data regarding the quantity of blood flow in the brain, and the changes in flow rate, may be vastly important in evaluating the risk of injury to the brain tissue and the efficacy of treatment. The availability of such data may enable the timely performance of various medical procedures to increase the cerebral blood flow, and prevent permanent damage to the brain.
Existing means for measuring cerebral blood flow are complex, expensive, and in some cases invasive, which limits their usefulness. Three non-portable methods that are presently used only in research are: 1) injecting radioactive xenon into the cervical carotid arteries and observing the radiation it emits as it spreads throughout the brain; 2) positron emission tomography, also based on the injection of radioactive material; and 3) magnetic resonance angiography, performed using a room-sized, expensive, magnetic resonance imaging system, and requiring several minutes to give results. A fourth method, trans-cranial Doppler (TCD) uses ultrasound and is not invasive, and gives immediate results. However, TCD fails in about 15% of patients, due to the difficulty of passing sound waves through the cranium, and it requires great skill by professionals who have undergone prolonged training and practice in performing the test and deciphering the results. Another disadvantage of TCD is that it measures only regional blood flow in the brain, and does not measure global blood flow.
Impedance measurements of the thorax are a known technique for monitoring intracellular and extracellular fluid in the lungs, in patients with congestive heart failure. This technique is effective because the resistive impedance of the thorax at low frequency depends on the volume of blood and other electrolytic fluids, which have a relatively high electrical conductivity, present outside cells. (The capacitive impedance of the thorax, on the other hand, depends largely on the volume of fluid inside cells.) A complicating effect in measuring the impedance of the thorax is the changing volume of air in the lungs during the breathing cycle, since air has a very high resistivity, and various methods have been developed to compensate for this effect. See, for example, U.S. Pat. Nos. 5,788,643, 5,749,369, and 5,746,214, the disclosures of which are incorporated herein by reference.
In these impedance measurements, current is often passed through the thorax with one set of electrodes, and a different set of electrodes is used to make voltage measurements. This “four wire” method essentially eliminates the voltage drop associated with the current flowing through any impedance in series with the thorax in the current-carrying circuit, for example due to poor contact (possibly changing unpredictably) between the current-carrying electrodes and the skin, or in the power supply producing the current. Those voltage drops, which are not of interest in measuring the impedance of the thorax, do not occur in the separate voltage-measuring circuit because it has high impedance and very little current flowing in it.
Photoplesthysmography is another technique used to monitor blood flow and blood volume, using the reflectivity of red or infrared light from the surface of the skin, for example the finger, or the earlobe. See, for example, J. Webster, “Measurement of Flow and Volume of Blood,” in John G. Webster (ed.), Medical Instrumentation: Application and Design (Wiley, 1997).
Magnetically inducing electrical fields in the body, including the head, is used in some existing medical procedures, principally for stimulation of the peripheral or central nervous system. See, for example, PCT publication WO 96/16692, the disclosure of which is incorporated herein by reference. Peripheral nerve stimulation is also a well known unwanted side effect of the time-varying magnetic fields used in magnetic resonance imaging.